1. Field of the Invention
The present invention relates to a spread spectrum communications system, and more particularly, to an apparatus for searching a multipath channel in a spread spectrum communications system, in which signals which are propagated through a multipath channel are received with different time delays, particularly with about 1 chip short-delays, and time delay information on each signal is searched for so that the time delay information can be allocated to a Rake receiver, and method thereof.
2. Description of the Related Art
In a spread spectrum communications system, when signals transmitted along a multipath channel are received by a receiver, the signals arrive at the receiver with different time delays. If the received signals are arranged and combined after compensating for the time delays, signals containing information become strong and noise signals containing no information become weak and therefore the performance of the receiver improves. To implement this, a multipath searcher for searching for different time delay information and a Rake receiver for arranging and combining signals using the time delay information are needed.
FIG. 1 is a structural diagram of an ordinary receiver. A radio front-end 100 converts a high frequency signal which is received through an antenna into a baseband signal and sends the baseband signal to a multipath searcher 130 and a Rake receiver 150. The signal demodulated in the Rake receiver 150 is input to a channel decoder for correcting errors which occurred in channels.
More specifically, the ordinary multipath searcher 130 includes a profile calculating unit 131 and a path selector 133. The profile calculating unit 131 calculates the power of a received information element so as to obtain the delay information of a signal which traveled along the multipath channel. In the spread spectrum communications system, a delay profile is obtained by using despread with respect to the delay time of a received signal. A delay profile can be obtained by calculating the power of an ordinary profile in order to remove the phase noise of the signal. The result of the calculated profile forms a shape in which a plurality of mountains gather together. The number of the peaks of the mountains indicates the number of paths in the multipath channel, and the distance between peaks indicates the time interval between paths of the multipath channel.
Meanwhile, the path selector 133 extracts time information for selecting paths of which signal power is strong among signals received through the multipath channel. The output of the profile calculating unit 131 is input to the path selector 133 and a peak higher than a predetermined value is selected. For selecting a peak, if the value of a current peak is greater than the values of the previous peak and the next peak, and greater than a predetermined value, then the current peak is selected. Time information elements of other peaks, each of which is calculated based on the peak having the highest value are sent to the Rake receiver 150.
Also, the Rake receiver 150 includes a plurality of fingers 151 through 155 for dispreading the received signals, and a combiner for combining despread data elements. The baseband signal, which is the output of the radio front-end 100, and the same number multipath delay information elements, which are output from the path selector 133, as the number of fingers, are input to the Rake receiver 150. Respective fingers 151 through 153 compensate for time delays due to the multipath channel, by compensating for received signals input from the radio front-end 100 according to the time information elements obtained in the path selector 133. Then, respective compensated signals are despread so that data is restored. The combiner 157 combines data elements despread in the fingers 151 through 155 into one, and makes signals containing information stronger and noise signals containing no information relatively weaker so that the performance of the receiver improves.
However, in a channel environment where an about 1 chip short-delay may occur, such as inside a building or in an urban area, the prior art multipath searcher cannot separate multipath signals which traveled along the multipath channel.